The present invention generally relates to electrochemical sensing techniques and devices thereof. The present invention also relates to techniques and devices thereof for measuring the concentration of gases in fluids. The present invention additionally relates to oxygen probes for measuring dissolved oxygen in fluids.
The concentration of dissolved oxygen (DO) in water-based systems is important either to assure adequate oxygen level in natural waters or to assure low dissolved oxygen in processes where dissolved oxygen can be expected to be corrosive. Dissolved oxygen is generally measured with an amperometric electrochemical sensor, whose signal is proportional to oxygen partial pressure. Because dissolved oxygen concentration is directly proportional to oxygen partial pressure at constant temperature, with a secondary measurement of water temperature, dissolved oxygen concentration in suitable units of measurement can be readily determined.
In a conventional oxygen probe, a thin gas-permeable membrane is utilized to isolate the water sample and the electrochemical cell. The electrochemical cell consists of at least two electrodes, one of which can be located internal to the thin gas-permeable membrane. This electrode is controlled by suitable means at a relatively negative potential compared to the second electrode. This electrode is often referred to as a cathode. Two or more internal electrodes can be immersed in an ionically conducting electrolyte. In operation, oxygen diffuses through the membrane from the sample side to the cathode, where it is electrochemically reduced to water. Hence, the oxygen partial pressure is zero at the membrane cathode interface and the difference in partial pressure across the membrane determines the oxygen flux. Measurement of probe current is directly proportional to membrane flux and oxygen partial pressure; with temperature measurement, probe current is suitably temperature compensated to allow for dissolved oxygen concentration computation and display.
One implementation of a dissolved oxygen probe is described in U.S. Pat. No. 4,076,596 to Connery et al., which is incorporated herein by reference. Connery et al. describes an apparatus for electrolytically determining a species in a fluid, including a method of use thereof.
Connery et al. generally describe an electrolytic cell for measuring the concentration of a species, such as oxygen. Depositing closed-spaced interleaved inert electrode surfaces on the surface of an insulating substrate and covering the electrode surfaces with a thin film of electrolyte and permeable membrane can construct the electrolytic cell. The electrolyte can be selected so the species being measured is generated at one electrode surface and consumed at the other with no net reaction in the electrolyte. Alternatively, closely winding two thin electrode wires about a cylindrical base and covering it with an electrolyte and a membrane may form a cylindrical configuration.
Dissolved oxygen can thus be measured in a liquid or fluid based on an amperometric sensor or probe in which oxygen gas from a measurement sample initially diffuses through a gas permeable membrane. Oxygen diffuses through the membrane into an electrolyte and is consumed at an electrode by electrochemical reduction to water at a working electrode, often referred to as a cathode. The chemical reaction that takes place can be represented by the following chemical formulation of equation (1):
O2+4H++4exe2x86x922H2Oxe2x80x83xe2x80x83(1) 
The driving force for oxygen diffusion through the membrane can be calculated by determining the difference in partial pressures across the membrane. In addition to that described in equation (1), a conventional dissolved oxygen (xe2x80x9cDOxe2x80x9d) probe can employ a chemically inert counter electrode, as described in U.S. Pat. No. 4,076,596. Such an oxygen probe, including later applications thereof, employ configurations in which the electrical current results from the reaction of equation (1) above passing through a companion counter electrode (or anode). This current is equal in magnitude to equation (1) but is of the opposite sign; and, hence, the reverse chemical reaction occurs at this counter electrode. The chemical reaction that can occur at this counter electrode can be represented by the following formulation of equation (2):
2H2Oxe2x86x92O2+4H++4exe2x80x83xe2x80x83(2) 
It is readily apparent that the sum of equations (1) and (2) does not correspond to any net chemical reaction. Because a net reaction does not result, reagents are not consumed. This method substantially reduces possible contributions of parameters that influence permeability, such as measurement sample stirring and membrane fouling. The electron flow illustrated by equations (1) and (2) above is thus directly proportional to oxygen partial pressure, which in turn is directly proportional to the oxygen concentration.
FIG. 1 depicts a prior art graph 10 illustrating normal dissolved oxygen probe operation at a controlled potential. Graph 10 illustrates half of the electrochemical fingerprint of oxygen dissolved in a conductive electrolyte within the DO probe. Only positive currents, corresponding to electrochemical reduction reactions are generally illustrated in FIG. 1. Because the actual probe current is a function of electrode geometry, current values are not displayed on the Y-axis 12 in graph 10. FIG. 1 indicates that DO can be reduced to water at potentials more negative than approximately xe2x88x920.1 V and that a potential window exists wherein current is independent of applied voltage. The curve 14 illustrated in FIG. 1 has a characteristic sigmoid shape with a plateau region 16, centered about xe2x88x920.6 V as indicated by reference numeral 19. This plateau region 16 corresponds to a limitation of reduction current because oxygen consumption is diffusion limited. As indicated at reference numeral 13, limiting current is generally proportional to oxygen partial pressure. Line 17 generally in FIG. 1 generally indicates a lower spec limit, while line 15 generally indicates an upper spec limit.
Because of the current limitation, it should be clear that probe current has little to no dependence on a reference electrode bias voltage as long as the controlled potential is located near the middle of the wave""s plateau. At all bias voltages at which oxygen is reducible to water, measured current is generally directly proportional to oxygen partial pressure. At potentials more negative than approximately xe2x88x920.9 V, sufficient energy is available to electrochemically reduce water to hydrogen gas. The slope 18 of the water reduction is quite steep, reflecting the large relative concentration of water. Thus, no diffusion limiting current is observed, based on graph 10 of FIG. 1.
FIG. 2 illustrates a prior art schematic diagram illustrating a DO probe 20 in controlled potential mode. FIG. 2 thus depicts a simplified representation of DO measurement electronics. A working electrode or cathode 23 of the DO probe 20 is generally connected to a transconductance amplifier 22 in which input voltage is maintained at a signal common potential and the amplifier output 29 is generally proportional to the current. Because node 25 is generally connected to a negative input of amplifier 22, this can result in the formulation Eo=xe2x88x92RfIin (xcex1 pO2) at amplifier output 29. Amplifier 22 is generally configured electronically in parallel with a resistor 25, labeled Rf in FIG. 2. The remaining amplifiers 24 and 26 indicated in FIG. 2 are composed of a reference electrode buffer (i.e., amplifier 24) and a control amplifier (i.e., amplifier 26). Amplifier 26 is generally connected to a voltage regulator 27. Amplifier 26 maintains the reference electrode +0.6 V above the signal common potential. Namely, in control, cathode 23 is at xe2x88x920.6 V compared to the reference electrode voltage, consistent with the current-voltage curves illustrated in FIG. 1. DO probe 20 further includes an anode 28 and a reference electrode 21. The prior art described above thus is capable of accommodating approximately 85% of measurement applications. Inherent to proper sensor function is the assumption that the current-voltage characteristics are as shown in FIG. 1.
There are two major exceptions, both related to specific applications, in which current-voltage curves significantly deviate from those illustrated in FIG. 1. The first exception is for those applications in which the reference electrode electromagnetic frequency (EMF) significantly changes. This can occur in applications containing sulfides and mercaptans. Because sulfides and mercaptans are gases with water solubility, these compounds are capable of diffusing through the gas permeable electrode membrane. Once inside the DO probe, these specific compounds will react with the silver ion from the probe""s reference electrode and precipitate because of their significantly lower solubility. The corresponding reduction of silver ions results in an approximate xe2x88x9260 mV shift for each decade reduction in active silver. Reference EMF shifts of xe2x88x920.3 V to xe2x88x920.6 V are common.
FIG. 3 depicts a prior art graph 30 illustrating DO probe operation at a controlled potential with xe2x88x920.4 V reference electrode EMF shift. Graph 30 of FIG. 3 generally illustrates the normal current-voltage curve, along with a xe2x88x920.4 V reference shifted (dotted) curve 32. On close examination of graph 30, it can be seen that the curve 32 contains the same information about measurement sample DO content. The oxygen and water reduction potentials, however, are shifted. While curve 32 contains the same information, current measurement at xe2x88x920.6 V can lead to serious errors. A failure to recognize the bias shift could lead the user to conclude that the DO probe is shorted, or the user might attempt to recalibrate the DO probe/analyzer only to find that the DO probe output is no longer directly proportional to dissolved oxygen. In FIG. 3, a current out-of-range value is indicated at reference numeral 33. Additionally a normal current value is indicated by reference numeral 35.
A second class of applications includes those involving acid gases, such as DO measurement in pressurized carbonated beverages. For example, carbon dioxide is a water-soluble gas, which is readily permeable through the probe membrane. Once inside, CO2 is not electrochemically active per se but does reduce the electrolyte pH. Because hydrogen ion is a reactant in the water reduction, as illustrated in FIG. 1, greater acidity decreases the external energy input requirements by approximately 60 mV/pH.
FIG. 4 illustrates a prior art graph 40 illustrating DO probe operation at a controlled potential with a xe2x88x925 pH electrolyte pH shift. Graph 40 of FIG. 4 generally illustrates the before and after current-voltage curves for a 5 pH acid shift. It should be noted that the leading edge of the oxygen reduction wave does not move, that water reduction commences at more positive potentials, and that the extent of the plateau is shrunk. A normal current reading is indicated by reference numeral 43 in FIG. 4. It should be further noted that absent any corrective action, higher current readings (i.e., see reference numeral 41) can be observed leading to measurement problems analogous to the reference shift described earlier.
Current-voltage shifts are known. An example of a conventional approach to address problems inherent with current-voltage shifts is employed in the Honeywell 7020 analyzer. This dedicated DO instrument has the capability of performing a current-voltage scan and displaying results, as indicated in FIGS. 1, 3 and 4 herein on an LCD dot matrix display. Approximately three minutes can be required to scan and display the current-voltage curve. It is then up to the operator to evaluate the displayed results and to select a new operating bias voltage. Thus, to overcome such application deficiencies of reference electrodes in DO probes, a need exists for an automated approach to determining dissolved oxygen bias voltage. A fully automated method and device thereof is not known to exist today. The present inventor thus believes that the present invention disclosed herein meets this important need.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide an improved electrochemical sensing method and system.
It is, therefore, another aspect of the present invention to provide an improved method and system for measuring the concentration of gases in fluids.
It is still another aspect of the present invention to provide an improved method and system for measuring oxygen dissolved in fluids.
It is yet another aspect of the present invention to provide an improved dissolved oxygen (DO) probe.
The above and other aspects can be achieved as is now described. A method and system for determining dissolved oxygen is disclosed herein. Instead of determining cathode current at controlled potential, it is possible to control cathode current and measure the corresponding reference electrode voltage. The reference electrode voltage is generally measured at a first current level and at a second current level utilizing the oxygen probe, wherein the first and second current levels define limitations of oxygen electrochemistry. An optimum electrode bias voltage can thereafter be automatically calculated based on the reference electrode voltage measured at the first current level and the second level to thereby provide accurate indications of dissolved oxygen thereof. Thus, one or more constant currents can be driven through amperometric type dissolved oxygen probes to develop reference electrode potentials defining the envelope of oxygen electrochemistry.
The oxygen probe can be configured to include at least one amplifier, which provides a constant current source sufficient to place the oxygen probe in the constant current mode. The amplifier can be, for example, a transconductance amplifier. The oxygen probe can be additionally configured to include at least two resistors electrically in parallel with another. At least one of the resistors is in electrical contact with an amplifier. The other of the two resistors can coupled to a cathode. The output of the amplifier can be connected to an anode. At least one input of an amplifier can be connected to a reference electrode. The cathode generally comprises a working electrode.
The oxygen probe disclosed herein is directed generally to a dissolved oxygen (DO) probe for measuring oxygen dissolved in a liquid. The present invention thus addresses problems associated with the momentary operation of an oxygen probe in a constant current mode, as opposed to a normal constant voltage mode. The reference electrode voltage is measured at two current levels, which define the limits of the oxygen electrochemistry. An optimum electrode bias can then be calculated and electronics associated with oxygen probe configured for normal DO measurement in a constant voltage mode.